1. Field of the InventionThis invention relates to emergency vehicle control systems for providing warnings of approaching emergency vehicles at intersections and more particularly relates to an emergency vehicle control system that utilizes traffic loop for preemptionof traffic signals at an intersection.2. Background InformationUsing existing technology current traffic loops are normally used to detect the presence of cars at an intersection. These traffic loop detectors activate and control the operation of traffic lights at intersections according to the approach ofvehicles.In the past decade, several approaches have been taken to provide traffic signal preemption for emergency vehicles. Existing systems use strobe lights to activate optical receivers at an intersection. Other systems use noise pattern recognitionto preempt based on approaching sirens. Recent systems have been developed using Global Positioning Systems (GPS); this has shown to be very effective in light metropolitan and rural areas. However each of these systems have drawbacks.The strobe phase preemption system has the drawback that an optical line of sight is required. Further the viewing angle of the optical receiver (problems with hills and turns) and range preemption is limited to a few hundred feet. Also thereceiver units and installation in vehicles are expensive.Noise pattern detection systems use siren noise detection and recognition for preemption. This is not advantageous because the direction of the sound is required. Also ambient noises can diminish the detection of siren noise such as traffic,horns, general traffic noises at intersections. Another drawback is that the siren noise recognition is of course severely limited by distances.GPS based preemption systems while effective also have some drawbacks. Such systems because they are very technical inherently require very expensive equipment. The system can also have difficulties because of vehicle position (buildings,b

United States Patent: 7113108
&nbsp;
( 1 of 1 )
United States Patent
7,113,108
Bachelder
, &nbsp; et al.
September 26, 2006
Emergency vehicle control system traffic loop preemption
Abstract
An emergency vehicle traffic signal preemption system using existing
inductive traffic loops that is either "car-active" or "car-passive". In
the "car-active" system, a passive element having position information
transmits an ID tag and the position information to a transceiver in the
vehicle when an emergency vehicle is detected by the existing inductive
traffic loop. In the "car-passive" system, a transceiver at the
intersection is activated to send an excitation signal to a transponder
on the emergency vehicle. The transponder responds with the emergency
vehicle ID. The transceiver in the vehicle in the "car-active" system or
the transceiver at the intersection in the "car-passive" system, transmit
position information to the traffic controller to preempt operation of
the traffic signals.
Inventors:
Bachelder; Aaron D. (Irvine, CA), Foster; Conrad F. (Los Angeles, CA)
Assignee:
California Institute of Technology
(Pasadena,
CA)
Appl. No.:
10/410,582
Filed:
April 8, 2003
Related U.S. Patent Documents
Application NumberFiling DatePatent NumberIssue Date
60371037Apr., 2002
Current U.S. Class:
340/906 ; 340/907; 340/931; 340/941
Current International Class:
G08G 1/07&nbsp(20060101)
Field of Search:
340/933,941,995.13,995.2,995.25
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Primary Examiner: Lee; Benjamin C.
Assistant Examiner: Tang; Son
Attorney, Agent or Firm: Christie, Parker & Hale, LLP
Parent Case Text
Priority of U.S. Provisional Application Ser. No. 60/371,037 filed Apr. 9,
2002 is hereby claimed.
Claims
What is claimed is:
1. A method for preempting traffic signals at an intersection comprising: detecting an emergency vehicle at a first intersection via an embedded inductive traffic loop; and
invoking a transmitter coupled to the traffic loop for transmitting position information to the emergency vehicle responsive to the detection of the emergency vehicle at the first intersection, the emergency vehicle forwarding the position information to
a traffic controller at a second intersection; wherein said traffic controller preempts traffic signals for controlling flow of traffic at the second intersection based on information on one or more positions transmitted by the emergency vehicle.
2. The method according to claim 1 including transmitting predictive intermediate position updates from said emergency vehicle to said traffic controller.
3. The method according to claim 2 comprising transmitting predictive intermediate position updates between intersections to said traffic controller.
4. The method according to claim 3 comprising transmitting predictive intermediate position updates between intersections that are determined by a dead-reckoning system.
5. The method of claim 1, wherein position of the emergency vehicle is determined based on the forwarded position information.
6. The method of claim 1, wherein the forwarded position information includes direction information.
7. A system for preempting traffic signals for controlling the passage of an emergency vehicle comprising: a traffic controller for controlling the operation of traffic signals at a second intersection; an inductive traffic loop at a first
intersection detecting presence of an emergency vehicle at the first intersection; a transmitter coupled to the traffic loop and transmitting position information to the emergency vehicle responsive to the detection of the emergency vehicle at the first
intersection; a transceiver in said emergency vehicle receiving the position information from said transmitter and forwarding said position information to said traffic controller at the second intersection; wherein said traffic controller preempts
traffic signals at the second intersection based on information on one or more positions transmitted by the emergency vehicle.
8. The system according to claim 7 wherein said transceiver transmits intermediate predictive position updates to said traffic controller.
9. The system according to claim 8 wherein said transceiver includes a dead-reckoning system, said intermediate predictive positions transmitted to said transceiver being determined by said dead-reckoning system.
10. The system according to claim 9, wherein said transceiver transmits a 4500 Mhz excitation signal to said transponder.
11. The system according to claim 10 in which said transponder transmits an emergency vehicle ID to the transceiver at a frequency of 900 Mhz.
12. The system of claim 7, wherein position of the emergency vehicle is determined based on the forwarded position information.
13. The system of claim 7, wherein the forwarded position information includes direction information.
14. A system for preempting traffic signals for controlling the passage of emergency vehicles comprising: a traffic controller for controlling the operation of traffic signals at a second intersection; an inductive traffic loop at a first
intersection detecting presence of an emergency vehicle at the first intersection; a transceiver coupled to the traffic loop, the transceiver being actuated to transmit a first signal in response to the detection of the emergency vehicle by said
inductive loop; a transponder on said emergency vehicle activated by the first signal transmitted by said transceiver and transmitting a second signal providing information about the emergency vehicle to said transceiver, said transceiver transmitting a
third signal providing position information of the emergency vehicle to said traffic controller; wherein said traffic controller preempts traffic signals at the second intersection based on the position information of the emergency vehicle.
15. The system of claim 14, wherein the position information includes direction information.
16. A method for preempting traffic signals at an intersection comprising: detecting an emergency vehicle at a first intersection via an inductive traffic loop; transmitting a first signal via a first transponder coupled to the traffic loop in
response to the detection of the emergency vehicle; activating a second transponder on the emergency vehicle based on the first signal and transmitting a second signal by the second transponder in response, the second signal providing information about
the emergency vehicle; and transmitting a third signal by the first transponder in response to the second signal, the third signal providing position information of the emergency vehicle to a traffic controller at a second intersection, wherein the
traffic controller preempts traffic signals at the second intersection based on the position information of the emergency vehicle.
17. The system of claim 16, wherein the position information includes direction information. Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to emergency vehicle control systems for providing warnings of approaching emergency vehicles at intersections and more particularly relates to an emergency vehicle control system that utilizes traffic loop for preemption
of traffic signals at an intersection.
2. Background Information
Using existing technology current traffic loops are normally used to detect the presence of cars at an intersection. These traffic loop detectors activate and control the operation of traffic lights at intersections according to the approach of
vehicles.
In the past decade, several approaches have been taken to provide traffic signal preemption for emergency vehicles. Existing systems use strobe lights to activate optical receivers at an intersection. Other systems use noise pattern recognition
to preempt based on approaching sirens. Recent systems have been developed using Global Positioning Systems (GPS); this has shown to be very effective in light metropolitan and rural areas. However each of these systems have drawbacks.
The strobe phase preemption system has the drawback that an optical line of sight is required. Further the viewing angle of the optical receiver (problems with hills and turns) and range preemption is limited to a few hundred feet. Also the
receiver units and installation in vehicles are expensive.
Noise pattern detection systems use siren noise detection and recognition for preemption. This is not advantageous because the direction of the sound is required. Also ambient noises can diminish the detection of siren noise such as traffic,
horns, general traffic noises at intersections. Another drawback is that the siren noise recognition is of course severely limited by distances.
GPS based preemption systems while effective also have some drawbacks. Such systems because they are very technical inherently require very expensive equipment. The system can also have difficulties because of vehicle position (buildings,
bridges, large cities, etc.) can occlude the signal. Further the system is entirely dependent upon GPS satellites and selected positioning modes.
Investigations have determined that GPS (when available) is very effective at the timing and determination of vehicle position. Original versions of software (and hardware) designed for highly accurate map-matching have precise location
determination of the emergency vehicle. The timing of the pedestrian and clearing phases at an intersection was incorporated into calculations of when to start preemption at any given intersection. There was concern that GPS-based system would not be
accurate enough based on the limited selected positional accuracy. However even when the selected position was still activated (at an accuracy of no more than .+-.50 meters), it was found the system could adapt effectively. Therefore the approach to
the preemption algorithm became more of a statistical calculation rather than a precise estimated-time-to-arrival calculation. In other words, given occasional accurate positioning the system could effectively compensate for large deviation between
accurate position locks.
One of the reasons the system could accept such large deviations in the system of the car was due to the appearance of normal behavior at any given intersection. When pedestrians and motorists are not aware that an intersection was preempted,
they simply didn't care. As long as a traffic light was returned to normal operation within approximately two minutes, this was seemingly no awareness of any problem. Given the infrequency of an emergency vehicle passing through a given intersection
throughout the day, the frequency that related delays would exceed two minutes is negligible. This allows loose margins on when to start preempting. In other words even if a system determines there is only a 50 percent probability that a vehicle is
going through an intersection, it could still preempt without noticeable disruption in traffic.
It is important to note that an emergency vehicle warning system such as that disclosed and described in U.S. Pat. Nos. 4,704,610 and 4,775,865 has two components: (1) the preemption of a traffic light and (2) a visible LED sign that alerts
motorists to oncoming emergency vehicles. If the LED sign is used in conjunction with the invention disclosed herein, the motorist is aware of an act of preemption. Thus, the two-minute limitation does not apply. The LED sign requires much more active
positioning to avoid "false warnings" (and likewise) "late-warnings".
In designing a preemptive system it was apparent that the system of the patents disclosed hereinabove would not function effectively in congested areas such as: large metropolitan cities, tunnels, and under bridges. When the lessons learned
about accuracy are applied to the GPS limitations, it is clear that a truly effective preemption system only requires accurate vehicle positioning at critical nodes in the system (nodes being key signal equipped intersections). Between intersections,
even using rough calculations based on dead reckoning, a system can produce highly effective predictions on when to preempt oncoming lights without causing unnecessary disruption in traffic flow.
Accordingly it is one object of the present invention to provide a traffic light preemption system for use in emergency vehicle warning system utilizing current traffic loops normally used to detect the presence of cars at an intersection.
Another object of the present invention is to provide a traffic signal preemption system utilizing existing traffic loops having a "car-active" system that relies on an on-board car computer to relay real-time vehicle positioning and travel
information to surrounding intersections.
Still another object of the present invention is to provide a traffic light preemption system utilizing existing traffic loops that is "car-passive" and relies on road-based detection and communications to identify vehicles as they pass.
BRIEF DESCRIPTION OF THE INVENTION
The purpose of the present invention is to provide a traffic light preemption system for use with existing emergency vehicle warning systems. The traffic light preemption system is efficient and economical because it is based on current traffic
loops under the road paving that are used to detect the presence of cars at an intersection which can be relied on to provide vehicle positioning information. The system disclosed herein may be used with the systems disclosed in U.S. Pat. Nos.
4,704,610 and 4,775,865 incorporated herein by reference.
Two types of systems have been designed to utilize existing traffic loops for vehicle positioning. One of these systems is a "car-active" traffic loop preemption system that uses a pass-through (transparent to normal behavior of the traffic
loop) element between the traffic loop and the traffic loop control box. Another system disclosed herein is a "car-passive" traffic loop preemption system that uses a passive RF transponder (no battery) about the size of a credit card that may be
affixed to the underside of the vehicle. The "car-active" traffic loop preemption system detects a car when it travels over existing or current traffic loops. The traffic loop activates the pass-through element, resulting in RF transmission of a tag
including position (in the form of latitude/longitude) and direction. Any "subscribing" vehicle within close proximity to the traffic loop receives the transmission.
The key innovative and unique technology disclosed herein is the application of current traffic loop positioning to emergency vehicle preemption of traffic lights. If a "subscribing" vehicle is an active emergency vehicle, a receiver in the
vehicle detects the tag and the car is given an accurate position at that precise moment. This position is forwarded to the neighboring intersections via transceiver on a real-time basis (1 Hz is the baseline frequency).
As the active emergency vehicle travels between intersections, a crude and inexpensive dead-reckoning system (simple compass and integrated speed) adequately updates the position of the vehicle. Since the system is aware of the road system
(using an on-board map-matching approach), it is discrete positioning problem (there are a limited subset of solutions to the problem). In layman's terms a car can only be on a street. As long as the error associated with the car's position is within
1/2 a street block, the system will function effectively for preemption purposes. Even if a vehicle's position on the correct street is off by 200+ feet in either direction, motorist's "lack of awareness" allow loose margins and early-bias preemption.
The key is to err on the side of adequate time for preemption.
Importantly, the use of hysteresis is critical to effective preemption behavior. Suppose the system determines that the car is statistically likely to come through a given intersection. Once preempted, the intersection must remain preempted for
an extended period of time regardless of whether the intersection receives additional "positive" preemptive signals from the same vehicle. In other words, once the statistical base for the decision to preempt exist, the system must sustain the
preemptive status until either:
A. The intersection receives a higher statistical weight of the same car coming (positive) transmission and position) and extends the preemption.
B. The reasonable, non-intrusive preemptive time expires when one minute is the baseline.
The "car-passive" traffic loop preemption system uses a passive RF transponder (no battery) about the size of a credit card which is fixed to the underside of the vehicle. The transponder is energized by a continuous wave 450-Mhz RF signal
generated by a power oscillator (also called an exciter). This power oscillator is linked to the existing inductive traffic loops which would act like a leaky transmission line. When excited by the signal from the power oscillator, the transponder
replies with its vehicle identification number (VIN) broadcast at a second frequency of 900 Mhz.
The overall traffic surveillance scheme disclosed has five major design parameters that may be traded against each other, namely, transceiver power, transceiver/transponder frequencies, transponder system efficiency, transponder sensitivity, and
transponder response time.
Transceiver power can be set near OSHA's maximum allowable level in order to achieve higher reliability. In order to distinguish transponder response from the transceiver excitation, two separate frequencies are used. These frequencies which
are somewhat constrained by the FCC's allocation of the band usage is chosen with the goal of miniaturization of the transponder--the higher the frequency, the smaller the conformal antenna and transponder. Additionally the transceiver/transmission
frequency must be set high enough to insure several redundant responses when the detected vehicle is over the traffic loop antenna. Transponder system efficiency is determined by the interaction of the conformal antenna, the harmonic generator, and the
embedded digital circuit. Transponder system efficiency is also moderated by the effectiveness of the coupling between the existing traffic loop antenna in the road (which has been designed for a different application) and the transponder antenna.
The preferred "car-passive" system provides singular activation points (intersection nodes) to surrounding intersections. Unlike the "car-active" system, there is no tracking or predictive analysis performed between these nodes. The
intersection themselves must be programed to preempt simply based on proximity. For conservative cities (bias towards normal, uninterrupted traffic) the algorithm at each intersection may only preempt if an adjoining (one light away) intersection
detects an active emergency vehicle coming in its direction. For a more liberal bias, the algorithm is constructed specific to the intersection itself. In other words, a main boulevard may trigger several lights ahead of any activation due to the high
likelihood the emergency vehicle would stay on high-speed route.
The "car-active" system provides a relatively high level of accuracy in comparison to the "car-passive" system as vehicle updates (based on tracking hardware) are provided every second. This translates into more efficient and flexible preemption
of traffic lights. However since the "car-active" system relies on vehicle transponders (on-board computer, transceiver, and battery connection), it requires significant cost added to the vehicle. Conversely, the "car-passive" system only requires a
simple passive element without any power which is much more economical.
It should be also noted that either system ("car-active" or "car-passive") can function in conjunction with a GPS-based system. If GPS is occluded (i.e., between buildings) or not functional, the system can "fall back" to the traffic loop,
dead-reckoning system disclosed herein. The two systems are complementary, especially when a city only wants to augment the minimal number of traffic loops.
The above and other objects, advantages, and novel features of the invention will be more fully understood from the following detailed description and the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating the "car-active" traffic loop preemption system according to the invention.
FIG. 2 is a diagram illustrating the "car-passive" traffic loop preemption system according to the invention.
FIG. 3 is a diagram of an intersection illustrating the general configuration of a preemption system using existing traffic loops according to the invention.
FIG. 4 is a schematic block diagram illustrating the general configuration of a traffic loop preemption system according to the invention.
FIG. 5 is a flow diagram of a program for use with the traffic loop preemption system of FIGS. 1 and 2.
FIGS. 6a through 6c are schematic layout diagrams of in-vehicle hardware and data flow traffic loop preemption systems of FIGS. 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
A "car-active" traffic loop preemption using a pass-through (transmitter to normal behavior of the traffic loop) element between the traffic loop and the traffic loop control box according to the invention is illustrated in FIG. 1. This traffic
loop preemption system is designed for use with the emergency vehicle warning and traffic control system such as that shown in U.S. Pat. Nos. 4,704,610 and 4,775,865 of Michael R. Smith et al issued Nov. 3, 1987 and Oct. 4, 1988, respectively and
incorporated herein by reference. The traffic light preemption system disclosed herein can be used with systems disclosed there, GPS systems, or as an adjunct to any of the systems available.
As shown in FIG. 1, when a vehicle 10 travels over inductive traffic loop 12 embedded in pavement 14 of a roadway, vehicle 10 will be detected. Inductive loop 12 also activates a pass-through element resulting in RF transmission of a tag
including position (in the form of latitude/longitude and direction). Any "subscribing" vehicle within close proximity of loop 12 receives the transmission.
An important, unique innovative aspect of the invention is the application of the inductive traffic loop 12 positioning to emergency vehicle 10 preemption of traffic signal(s) 16 at an intersection. If an active emergency vehicle 10, shown in a
number of different positions in FIG. 1, is a "subscribing" vehicle, a transceiver (not shown) in the vehicle detects a tag and the vehicle is given an accurate position at that precise moment. This position is forwarded (i.e., transmitted) as indicated
at 18 to neighboring intersections via the emergency vehicle transceiver on a real-time basis (1 Hz is a baseline frequency).
As the active emergency vehicle 10 travels between intersections, a basic and inexpensive dead-reckoning system (a simple compass and integrated speed) adequately updates the position of the vehicle. Since the system is aware of the road system
(using an on-board map-matching approach), it is a discrete positioning problem (there are a limited subject of solutions to the problem). In layman's terms, vehicle 10 can only be on a street or roadway 14. As long as the error associated with the
position of vehicle 10 is within a one half street block, the system will function effectively for preemption purposes. Even if the position of vehicle 10 on the correct street is offered by 200+ feet in either direction, motorists "lack of awareness"
allow loose margin and early biased preemption. The key is to error on the side of adequate time for preemption of traffic light 16.
A "car-active" traffic loop preemption system is illustrated generally in FIG. 1. The inductive traffic loop preemption uses a pass-through element between inductive traffic loop 12 and traffic loop control box 20. When vehicle 10 travels over
inductive traffic loop 12, the vehicle is detected and a fixed position is obtained. Likewise inductive loop 12 also activates pass-through passive element 18 that results in an RF transmission of a tag indicated at 22 including position (in the form of
latitude/longitude and direction) to a transceiver (not shown) in vehicle 10. Any subscribing vehicle 10 within close proximity of inductive loop 12 receives a transmission.
Since vehicle 10 is a "subscribing" active emergency vehicle, transceiver in vehicle 10 detects the ID tag transmitted by passive element 18 and is given an accurate position e.sub.1 at that precise moment. This fixed position is forwarded to
traffic controller 24 and neighboring intersections via the emergency vehicle transceiver on a real-time basis (1 Hz is the baseline frequency). Predictive position updates (e.sub.2 and e.sub.3) from vehicle 10 are also transmitted to traffic controller
24 as indicated at 28 and 30. The intermediate predictive position is determined by a dead-reckoning system. At the next intersection 32 another fixed position e.sub.1 is obtained by an inductive traffic loop (not shown) at that intersection with a
subsequent predictive position update e.sub.2 being transmitted. Traffic controller 24 therefore preempts the traffic light 16 at intersection 36 as emergency vehicle 10 approaches.
A "car-passive" traffic loop preemption using a passive RF transponder (no battery) about the size of a credit card is affixed to emergency vehicle 10. The transponder in vehicle 10 is energized by continuous 450 Mhz RF signal generated by power
oscillator 19 (also called an exciter). Power oscillator 19 is connected to existing inductive traffic loop 12 which acts like a leaky transmission line. When excited by the signal from power oscillator 19 the transponder in vehicle 10 replies as
indicated at 21 with its vehicle identification number (VIN) broadcast at a second frequency of 900 Mhz. The position of vehicle 10 is updated by transmissions from traffic loop boxes 36 and 38 only when a car passes an intersection. Thus a traffic
controller 24 is constantly updated as vehicle 10 travels along roadway 14 but there are no intermediate updates.
The general configuration and layout at the intersection having traffic controller 24 for controlling the operation of traffic signal(s) 16 is illustrated in FIG. 1. The emergency vehicle warning system disclosed and described in the patents
referred hereinabove includes an emergency warning sign 40 activated and controlled by traffic controller 24. Emergency warning sign 40 indicates the flow of emergency vehicles along roadways 42 and 44 while traffic controller 24 controls operation of
the traffic signals at the intersection. Traffic loop circuit 46 transmits an exciter signal to vehicle 10 and receives a transmission signal with the emergency vehicle ID. Transmission to and from an emergency vehicle 10 are piggybacked on the
inductive traffic loop. A vehicle detected by traffic loops 12 is detected by inbound traffic loop box 48 and transmitted over vehicle detect enable line 50 to traffic loop circuit 46.
The diagram in FIG. 3 shows the traffic loop layout configuration. The road embedded inductive traffic loop circuit 46 is a module that includes a signal condition, receiver and transmitter. All transmissions are received/sent via piggyback
along inductive traffic loop hard-line assembly and the loop itself. The system illustrated is enabled anytime a vehicle is detected over inductive traffic loop 12 and all transmissions are low power to limit the distance of decoded transmission. In a
"car-active" configuration, a longitude and latitude pair are provided to an emergency vehicle 10. In a "car-passive" configuration, vehicle 10 reflects (via exciter) its ID back to the active element of traffic loop circuit 46 at the intersection.
The inductive traffic loop intersection hardware/data layout is illustrated in FIG. 4. This diagram illustrates the general hardware layout and data flow at each intersection. Each traffic loop 50 is attached to a primary conditioning box 52
along with an embedded preemption module 54. In a "car-passive" configuration, exciter system transmitter 56 sends an exciter signal to traffic loop 50 to energize passive system receiver 58 on the vehicle (and get an ID tag). In a "car-active"
configuration, position information is transmitted via the loop. All preempt module 54 are connected to a central preempt controller 60 in the intersection traffic controller cabinet 62. Central preempt controller 60 is responsible for immediate
preemption at the local intersection, forwarding position/ID information of triggering emergency vehicles to neighboring intersections and receiving/processing an external trigger from neighboring intersections. The notification to neighboring
intersections and from neighboring intersections is through medium range transceiver 64.
A traffic loop intersection system program flow diagram is illustrated in FIG. 5. This flow diagram outlines a combined functionality, logic-tree, and program of both embedded road units and cabinet preemption controller 60. The unique feature
of the invention is the use of existing inductive traffic loops as both an activation device and localized antenna to obtain sufficiently accurate location information.
Traffic loop in-vehicle hardware/data layout is illustrated in FIGS. 6a and 6c which show possible vehicle configurations for the traffic loop preemption system disclosed herein. In the "car-passive" system, FIG. 6a illustrates the use of a
short-range transmitter 62 powered by car battery 64. Short-range transmitter 62 transmit the vehicle ID to the traffic loop circuit. FIG. 6b illustrates a non-powered design where an RF inductive power supply 66 provides an output to short-range
transmitter 62 which again transmits vehicle ID continuously or when pinged by the loop, respectively. This simple design makes a vehicle transponder extremely inexpensive and easy to install.
A third implementation of a vehicle system incorporates on-board-dead reckoning capability. When a vehicle passes a traffic loop, it receives either intersection ID to be looked up in database 68 or a latitude/longitude location. Each loop is
used to "snap" a positive fixed location for dead-reckoning microcontroller receiving inputs from the optional intersectional database or latitude/longitude navigation inputs and heading accelerometer 72. Dead-reckoning microcontroller 70 continues to
use additional onboard navigation data to estimate future positions. In the "car-active" system, vehicle 10 continues to broadcast its "best known" position to every neighboring intersection via medium range RF transmitter 74. As in the "car-passive"
system, vehicle ID is still sent to each traffic loop vehicle tag transmitter 76 for recording and redundancy.
It is important to note that either system can function in conjunction with a GPS based system. If the GPS system is occluded or not functional, the system can "fall back" to the traffic-loop dead-reckoning system. The two systems are
complementary, especially when a city only wants to augment the minimal number of traffic loops.
Thus there has been disclosed a novel traffic signal preemption system utilizing existing inductor traffic loops. The inductive traffic loops are already installed in most cities, with a diameter of about six feet providing a reliable detection
of activity across a 10 ft. traffic lane. These six foot inductive traffic loops comprised of about 90 percent of the existing loop infrastructure. An advantage of using existing inductive loops is because they are insensitive to surface accumulation
of water, ice, snow, mud, etc. The use of existing technology in the road and optionally using simple dead-reckoning equipment on emergency vehicles themselves, the system can insure the provision of accurate direction, distance, and robot preemption in
highly congested areas. In the "car-active" system, accurate fixed position and intermediate predictor positions are continuously transmitted to a traffic controller. In the "car-passive" system, a transponder reacts to excitation from a continuous
wave 450 Mhz RF signal generated by a power oscillator or exciter that responds with its VIN at a second frequency of 900 Mhz.
This invention is not to be limited by the embodiment shown in the drawings and described in the description which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.
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